This invention relates generally to optical communication. More particularly, it relates to optical switches for wavelength division multiplexing.
Optical wavelength division multiplexing (WDM) has gradually become the standard backbone network for fiber optic communication systems. WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology.
Despite the substantially higher fiber bandwidth utilization provided by WDM technology, a number of serious problems must be overcome, such as multiplexing, demultiplexing, and routing optical signals, if these systems are to become commercially viable. Present WDM systems are typically capable of isolating signals separated by about 100 GHz only with great difficulty. Isolation is presently not practical for optical signals separated by about 50 GHz or less.
The problem is illustrated by FIG. 1A, which depicts transmission attenuation versus frequency. A conventional filter typically has a passband 1 centered on a center frequency (or wavelength). The filter shown in FIG. 1 is a periodic filter in that the pass band repeats periodically over a range of frequencies. The passband is centered on the frequency of one or more even channels 2 and 4. Unfortunately, passband 1 also overlaps an odd channel 3. Because of this overlap some of the energy from odd channel 3 is transmitted by passband 1. This phenomenon is sometimes referred to as crosstalk. Ideally the filter should have a passband with a trapezoidal shape, like dashed curve 5, which provides about 30 dB of attenuation where the passband crosses the even channel. The isolation may be improved, as shown in FIG. 1B, by passing the optical signal through two filters having passbands centered on the same frequency. The resulting passband 6 is narrower than passband 1, but not as narrow as the ideal passband 5 of FIG. 1A. The attenuation is only about 20 dB where passband 6 crosses odd channel 3.
U.S. Pat. No. 5,946,116, issued to Wu et al. on Aug. 31, 1999 and incorporated herein by reference, describes structures for realizing optical switches (routers) that achieve very high extinction ratio operation. However, these switches are wavelength independent. U.S. Pat. No. 5,867,291, issued to Wu et al. and incorporated herein by reference, discloses systems that provide the functions of wavelength de-multiplexing and routing. However, this single stage design relies primarily on the filter design. The transmission function of the filter has to be close to an ideal square with a flat top to realize the desired low crosstalk operation. U.S. Pat. No. 5,694,233, issued to Wu et al. and incorporated herein by reference, discloses a combination of the two architectures and concepts presented in the above-cited patents to create a switchable wavelength router. This structure employs double stage filters that can obtain a better (purified) pass-band. Unfortunately, the performance of this device is less than ideal.
Another conventional filter design creates a shaped spectral response by sandwiching birefringent material (e.g., birefringent crystals) between two polarizers. Unfortunately, the polarizers tend to waste optical energy by absorbing radiation having the xe2x80x9cwrongxe2x80x9d polarization. U.S. Pat. No. 5,867,291, issued Feb. 2, 1999 to Wu et al. describes a filter design that creates a shaped spectral response by sandwiching a polarization rotator and a stack of birefringent waveplates having selected orientations between two birefringent polarizers. The combination acts as a polarization interference filter that selectively passes a selected set of frequencies with a horizontal polarization and a complementary set of frequencies with vertical polarization. Ideally this filter has a trapezoidal spectral response curve with a flat top. A flat top is important when each band is intended to transmit several WDM channels or to maintain the signal shape when the channels are so dense that the passband width is comparable to the signal width. By increasing the sampling points or the number of waveplates a better transmission function that more closely approximates a trapezoidal transmission with steep transitions is obtained. Theoretically this transmission function can have a perfectly trapezoidal shape in the desired spectral bandwidth. Minimum side slopes, 100% transmission, and flat top response are possible. Practically, however, the physical size limits the number of stages a practical device will sacrifice some of the features such as ripple on the top, shallower slope, and side lobe fluctuation.
There is a need, therefore, for an improved polarization filter that overcomes the above difficulties.
Accordingly, it is a primary object of the present invention to provide a method improving optical isolation by trimming the spectra of signals. It is a further object of the invention to provide a means for flattening the transmission spectrum of an optical signal. It is an additional object of the invention to provide an optical switching apparatus incorporating trimming and flattening filters.
These objects and advantages are attained by a wavelength slicing apparatus and method which includes two filters having passbands offset from each other and from a signal channel. The offset passbands reduce transmission where they do not overlap such that transmission of optical signals in a neighboring channel is suppressed. A third filter having a passband centered on the signal channel frequency may be included to enhance optical signal isolation. In one embodiment one or more of the filters selectively rotates a polarization of a portion of the signal spectrum, and passes the signal through a polarizer oriented to attenuate either the rotated portion or a non-rotated portion. The wavelength slicing techniques described herein may be used for channel isolation or passband flattening.
Wavelength slicing can be incorporated into an optical switching method to improve optical isolation between channels. In the switching method, an input optical signal is spatially separated into first and second optical signals having mutually orthogonal polarizations. The two signals are then each separated into two or more components having characteristic spectra. Polarizations of selected portions of the spectra are rotated. The components then pass through a polarizer to trim or flatten their spectra. At least one of the components is then selectively coupled to at least one of two or more outputs.
A switching apparatus incorporating wavelength-slicing elements may implement embodiments of the switching method. The apparatus generally comprises a birefringent walk-off device, a wavelength filter, a wavelength trimmer, and a flattening filter. The wavelength filter separates an input signal into two or more components each characterized by a spectrum. The wavelength trimmer typically comprises a birefringent crystal beamsplitter and a first waveplate. The wavelength trimmer typically comprises a second waveplate and a linear polarizer. The waveplate is configured to rotate certain portions, e.g., the leading or trailing edge of the spectra of the components by 90xc2x0 while leaving polarizations of the center portions substantially unrotated. The polarizer is oriented to extinguish to rotated components and transmit the unrotated components. The flattening filter preferentially attenuates the peaks of the spectra to achieve a flat-topped transmission. The flattening filter may comprise a polarization rotator and a linear polarizer. The polarization rotator is configured to rotate certain portions of the spectra of the components, e.g. the peaks of spectra, by less than 90xc2x0. The polarizer is typically oriented to attenuate to rotated components to flatten the spectra.